Coal liquefaction processes have been developed for converting coal to a liquid fuel product. For example, U.S. Pat. No. 3,884,794 to Bull et al discloses a solvent refined coal process for producing reduced or low ash hydrocarbonaceous solid fuel and hydrocarbonaceous distillate liquid fuel from ash-containing raw feed coal in which a slurry of feed coal and recycle solvent is passed through a preheater and dissolver in sequence in the presence of hydrogen, solvent and recycled coal minerals, which increase the liquid product yield.
Although broad ranges of temperature, hydrogen partial pressure, residence time and ash recycle are disclosed, it has been generally believed that commercially workable conditions for achieving the highest total liquid yields involve a hydrogen partial pressure of about 2,000 psi, a slurry residence time of about 1 hour and the use of about 7 weight percent recycle ash in the slurry feed, while achieving a total liquid yield of approximately 35 to 40 weight percent based upon MAF feed coal.